In this universe, your soul acts as your energy/life force. When your soul runs out, so to speak, you simply die.

However, there are individuals that possess both an excess of energy/life force and the ability to control that excess. This ability--KNOWN OTHERWISE AS MAGIC (will get a better name)-- manifests itself most noticeably as strange coloured particles in a person's eyes. It is believed that the colour of the particles, is the colour of your soul. MMM SYMBOLISM